α-sialon is a compound represented by a general formula MxSi12−(m+n)Alm+nOnN16−n, which is provided by an α-type silicon nitride crystal including Al and O dissolved therein in a substitutional solid state and a metal element M dissolved therein in an interstitial solid state.
x, m, and n are parameters which determine the dissolved amounts of the applicable elements in solid states (i.e., the composition of the sialon), and take values in the ranges of 0<x≦2, 0<m≦6, and 0<n≦3, respectively.
Known examples of a metal element M forming an α-sialon include: Li, Mg, Ca, Sr, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and the like. α-sialon sintered bodies are excellent in heat resistance, corrosion resistance, and the like, and are thus used as cutting tools, heat-resistant machine parts, and the like.
Further, there is recently expected a usage of α-sialon as a phosphor (see patent-related reference 1) which is provided by dissolving a photoactive metal such as Eu in a lattice of α-sialon in a solid state so that the resultant solid solution is excited by ultraviolet light or blue light to thereby emit visible light.
Examples of typical production methods of α-sialon include a reactive sintering method for molding a mixed powder of a silicon nitride powder, an aluminum nitride powder, and oxides of metal M, and heating the molding to 1,700 to 1,800° C., to thereby obtain a solid sintered body. Also known as a synthesis method of powdery α-sialon, is a carbothermal reduction-nitridation method (see patent-related reference 2) for adding a carbon powder into silicon oxide, aluminum oxide, and oxide of metal M, and for heating the resultant mixture to 1,400 to 1,700° C. within a nitrogen atmosphere, to thereby obtain a powder.
However, to be obtained by the reactive sintering method is not a fine powder, but a dense sintered body. In turn, although there can be obtained a fine powder of α-sialon having a particle size of 0.1 to 2 μm by the carbothermal reduction-nitridation method, the added carbon is left in a final product to thereby cause coloration, which is undesirable for optical usage.
Known as a synthesis method of a high purity nitride powder other than α-sialon, is one (see patent-related reference 3) for reducing and nitriding an aluminum oxide powder in a mixed gas of ammonia and hydrocarbon, thereby obtaining an aluminum nitride powder.
There has been further reported a method (see patent-related reference 4) for reducing and nitriding a silicon oxide powder within a mixed gas of ammonia and hydrocarbon, thereby obtaining a silicon nitride powder.
However, there have not been yet established methods for synthesizing powders which are highly pure in nature, fine in size, and less in impurities, insofar as concerned with α-sialon.
[Patent-Related References]
Patent-related reference 1: JP-A-2002-363554
Patent-related reference 2: JP-4-60050
Patent-related reference 3: JP-A-2002-97006
Patent-related reference 4: JP-7-91043